1. Field of Invention
The present invention relates to a method and a device for estimating the parameters of a propagation channel, so as to decode digital signals, by using a learning sequence or reference sequence known to the receiver. The signals are sent in successive frames, each frame being partitioned into a specified number of time slots.
The invention applies for example for demodulating a broadband or spread spectrum digital signal.
The invention finds its application in the field of cellular networks operating on the “CDMA” principle (the abbreviation standing for Code Division Multiple Access), such as the networks based on the IS95 standard which has been in service for several years in the USA, or else those based on the UMTS standard (the abbreviation standing for Universal Mobile Telecommunication System) which ought to be operational in Europe at least by the year 2002. It applies in particular within the framework of mobile radio communications.
The method according to the invention can without departing from the scope of the invention be applied to propagation channels of any nature.
2. Description of Related Art
In a CDMA system all the users share the same band and use waveforms which are spectrally spread with the aid of codes, each user having his own code.
By way of indication, a description of such a system can be found in one of the following references:    [2] W.C.Y. LEE, “Overview of cellular CDMA”, IEEE Transactions on vehicular technology, vol. 40, No. 2, May 1991 or    [3] R. L. PICKHOLTZ, D. L. SHILLING and L. B. MILSTEIN, “Theory of spread-spectrum communications—A tutorial”, IEEE Transactions on communications, vol. com-30, No. 5, pp 855–884, May 1982.
The principle of direct sequence spread spectrum is to chop a useful bit of duration T into NG pulses or chips of duration T′=T/NG. The band W′ of the resulting signal is thus NG times larger than that of the useful signal W, thereby theoretically allowing NG times as large a modulation rate. The pulse sequences or codes, are generally pseudo-random sequences (PN sequences), designed to have particular autocorrelation and cross-correlation properties (ideally, the autocorrelation is pulse-like and the correlations between codes are zero). Such sequences are described for example in the two works: [1]: J. G. PROAKIS, “Digital communications”, McGraw Hill and the aforesaid reference [2].
On reception, each user (or transmission channel) is demodulated by an operation of correlation with its spreading code, known to the receiver. This processing is a matched filtering which imparts a reception gain equal to the ratio of the spread band to the initial useful band represented in FIG. 1.
The CDMA concept allows not only multiple access (large combinatorial offered by pseudo-random codes, for allocating a different code to each user) but also increased robustness to scrambling (intentional or otherwise) and to poor propagation conditions (multipaths, fading, etc).
At present, it is known to use a spectrally spread waveform in order to effectively counter the phenomenon of multipaths (this phenomenon occurs for various propagation channels and in particular for the mobile radio channel) and even to profit therefrom in order to combat fading (random variations in the amplitude and phase of the signal).
Specifically, if Tm is the temporal spreading of the channel and if W is the bandwidth of the signal transmitted, then it is theoretically possible to isolate in reception TmW components (a bandwidth W makes it possible in reception, by correlation, to isolate paths with a temporal resolution equal to 1/W), which correspond physically to the various propagation paths. All or some of these components are independent and it is possible to recombine them in such a way as to increase or maximize the signal-to-noise ratio. A diversity effect is thus obtained, exploiting the statistical independence of the various propagation paths making up the signal received.
This diversity technique was initially proposed by Price and Green in 1958 who gave the name RAKE receiver to the optimal processing (the receiver isolates each path and then undertakes a recombination, hence the analogy with a garden rake). This processing, which coherently sums all the paths, consists simply of a filtering matched to the signal received, that is to say matched to the channel (it may be shown that this processing is optimal provided one considers the reception of a single user or code, all the other users constituting noise possessing the property of being white and gaussian).
In this regard, reference may be made for example to the work “Digital communications” published by McGraw Hill and authored by J. G. Proakis.
The implementation of the RAKE receiver therefore assumes that the impulse response of the channel, that is to say the complex amplitudes of all the paths, is known. In practice, it is possible to employ an estimate of these amplitudes which takes account of the various characteristics of the propagation channel (temporal spreading, fading) and in particular of its various scales of fluctuation (fast fading and slow fading).
On the mobile radio channel, two scales of variation of the propagation characteristics are generally distinguished, described for example in the work authored by D. Parsons and published by Pentech Press in 1992 under the title “The mobile radio propagation channel”.
The first scale corresponds to fast fading (or Rayleigh fading), which induces a variation in the amplitudes and phases of the paths after a time of the order of the channel coherence time (the inverse of the Doppler band of the channel). In the mobile radio context, this decorrelation time (time interval for which two realizations of the complex amplitude of a path are decorrelated) corresponds to an advance of the mobile of the order of half the wavelength of the carrier frequency reference [4], On this timescale, the delay times of the paths are constant.
The second scale of fluctuation corresponds to slow fading (or long-term fading), due to the masking effects and to the changes of environment. This phenomenon induces on the one hand a modification of the average power of the paths, and on the other hand a significant alteration in the delay times of the paths; some paths are also liable to appear and others to disappear.
The decorrelation time (or the decorrelation distance) associated with slow fading depends on the channel, that is to say on the type of environment in which the mobile is deploying. In practice, this decorrelation time is markedly greater (at least an order of magnitude) than that associated with fast fading. For example, the ETSI within the framework of UMTS, indicates a slow fading decorrelation distance, for an urban environment, of around 20 meters; this leads to a ratio of around 300 (20/(λ/2) with λ=c/f and f=2 109 Hz) between the long-term and short-term decorrelation times.
The subject of the invention relates to a method allowing dynamic estimation of the propagation channel, taking account of the various fluctuations alluded to above, with the aim of demodulating a digital signal.
The method is applied in particular in the following cases:                Single-user reception, that is to say when interested in the demodulation of just one user, all the others being regarded as noise (no joint detection or multiuser detection);        Transmission of a reference sequence, in addition to the useful data, which is dedicated to the estimation of the propagation channel;        The propagation channel is fluctuating;        The temporal spreading of the channel is less than the duration of a useful bit. Thus, the inter-symbol interference can be neglected and the implementation of an equalizer downstream of the RAKE receiver is not necessary. Each bit or useful symbol is demodulated independently of the other bits or useful symbols;        The channel can be regarded as constant over the duration of a bit or useful symbol (Tcoherence>>T, where Tcoherence is the channel coherence time).        
The word “symbol” designates a unit of information, the expression “learning symbol” or “pilot” corresponds to a unit of information of the known signal.